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Rivier College CS575: Advanced LANs Chapter 15: LAN Performance

Rivier College CS575: Advanced LANs Chapter 15: LAN Performance. LAN Performance Overview. Measures of Performance Effect of Propagation Delay and Transmission Rate Factors That Affects Performance Bounds of Performance Comparative Performance of Token Passing and CSMA/CD

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Rivier College CS575: Advanced LANs Chapter 15: LAN Performance

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  1. Rivier CollegeCS575: Advanced LANsChapter 15: LAN Performance LAN Performance

  2. LAN Performance Overview • Measures of Performance • Effect of Propagation Delay and Transmission Rate • Factors That Affects Performance • Bounds of Performance • Comparative Performance of Token Passing and CSMA/CD • Behavior of Contention Protocols LAN Performance

  3. Measures of Performance • [D]: The delay that occurs between the time a packet or frame is ready for transmission from a node, and the completion of successful transmission. • [S]: The throughput of the LAN; the total rate of data being transmitted between nodes (carried load). • [U]: The utilization of the LAN medium; the fraction of total capacity being used. LAN Performance

  4. Ideal Channel Utilization LAN Performance

  5. Measurements of Performance LAN Performance

  6. Representative Values of a and U a = Propagation time/Transmission time; Maximum possible utilization, U = Throughput/Data rate = 1/(1 + a) LAN Performance

  7. The Effect of a on Utilization for Baseband Bus LAN Performance

  8. Utilization U as a Function of a LAN Performance

  9. Effect of a on Throughput S LAN Performance

  10. Overhead • Administrative Overhead for controlling protocols includes address and synchronization bits. • In contention protocols (ALOHA, S-ALOHA, CSMA, CSMA/CD): Time wasted due to collisions; need for acknowledgement frames. S-ALOHA requires that slot size equals transmission plus maximum propagation time. • In Token Ring: Time waiting for token if intervening stations have no data to send. • In Token Bus: Time waiting for token if logically intervening stations have no data to send; token transmission; acknowledgement frames. • Explicit reservation: Reservation transmission, acknowledgements. • Implicit reservation: Overhead of protocol used to establish reservation, acknowledgements. LAN Performance

  11. LAN Performance: Bounds on Performance • Regions, based on the magnitude of the offered load: 1. A region of low delay through the network, where the capacity is more than adequate to handle the load offered.2. A region of high delay, where the network becomes a bottleneck.3. A region of unbounded delay, where the offered load exceeds the total capacity of the system. • Maximum possible throughput: THRU < Nactive/(Tidle + Tmsg) • The breakpoint between these two bounds occurs at: Nactive/(Tidle + Tmsg) = 1/ Tmsg or Nactive= (Tidle + Tmsg)/Tmsg • For a lower bound on delay: Tdelay > Tmsg or Tdelay > NactiveTmsg - Tidle LAN Performance

  12. Performance of Token Passing and CSMA/CD • For Token Ring: Throughput S = 1/(1 + a/N) @ a < 1 and S = 1 @ N >> 1, a < 1 S = 1/a(1 + a/N) @ a > 1 and S = 1/a @ N >> 1, a > 1 Delay D = N + a – 1 @ a < 1 D = aN @ a >1 • For CSMA/CD: S = 1/[1 + 2a(1-A)/A] and S = 1/(1 + 3.44a) @ N >> 1; A = (1 – 1/N)^(N – 1) Delay for CSMA/CD is difficult to express and depends on the exact nature of the protocol (persistence, retry policy). LAN Performance

  13. Performance Analysis Conclusions • For the given parameters, the smaller the mean frame length, the greater the difference in maximum mean throughput rate between token passing and CSMA/CD. This reflects the strong dependence of CSMA/CD on a. • Token ring is the least sensitive to workload. • CSMA/CD offers the shortest delay under light load, while it is most sensitive under heavy load to the workload. LAN Performance

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